It has been recently and intensively studied that a nitride of a group 13 element such as gallium nitride is used to produce a semiconductor device such as a blue ray laser, white ray laser, blue-violet ray semiconductor laser and the like and that the device is applied to various kinds of electronic appliances.
Such prior gallium nitride-based semiconductor device has been mainly produced by vapor phase process. Specifically, it has been produced by growing a thin film of gallium nitride by heteroepitaxial growth on a sapphire or silicon carbide substrate by organic metal vapor phase deposition (MOCVD) or the like. In this case, the substrate and thin film of gallium nitride are different from each other in thermal expansion coefficient and lattice constant, so that dislocation (a kind of lattice defect of crystal) is generated in high density in the grown gallium nitride. It has been thus difficult to obtain gallium nitride of a low dislocation density and high quality according to the vapor phase process.
Thus, according to Japanese patent Publication No. 2002-217116A, an under layer of GaN single crystal or the like is formed on a seed crystal substrate by vapor phase process, and an over layer of GaN or the like is then formed again on the under layer. It is generated a region of void or indium precipitation along an interface of the under and over layers so that it is tried to reduce threading dislocation from the under layer to the over layer.
On the other hand, it has been developed liquid phase process in addition to the vapor phase process. So called flux method is one of the liquid phase processes. In the case of gallium nitride, sodium metal is used as a flux so that it is possible to lower temperature required for crystal growth of gallium nitride to around 800° C. and pressure to several MPa. Specifically, nitrogen gas is dissolved into mixed melt of the sodium metal and gallium metal so that gallium nitride is crystallized and grown in the melt in the supersaturating state. According to such kind of liquid phase process, the dislocation can be reduced compared with the vapor phase process, so that it is possible to obtain gallium nitride having a low dislocation density and high quality.
Such flux process has been also extensively researched and developed. For example, according to Japanese Patent Publication No. 2005-263622A, the speed of crystal growth of gallium nitride in the direction of thickness (direction of C-axis) is as low as about 10 μm/h and ununiformed nucleation tends to occur along liquid-vapor interface of the melt in prior flux method. It is thus disclosed a method of producing gallium nitride for overcoming such problems.
The applicant filed Japanese Patent Publication No. 2010-168236A and described the correlation between power of agitation and generation of inclusion. According to the Patent document, it was disclosed to control the growth rate in a preferred range and to adjust the speed and inversion condition of rotation of a crucible, for growing a crystal free of the inclusions.